1. Field of the Invention
This invention relates to a method of making precursors and products of structural metal matrix composites having a long-fibre ceramic reinforcement, for use in applications requiring improved damage tolerance compared with current metal matrix composites.
The term "structural composite" is used herein in the context of material applications where suitability for function is judged on the physical strength and toughness properties such as tensile strength, proof stress, elastic modulus, ductility and damage tolerance and possibly density and resistance to corrosion under service conditions.
2. Discussion of Prior Art
Ceramic reinforcement fibres are used in metal matrix composites for structural applications in order to add strength or stiffness to the matrix metal. Such structural composite materials are of potential interest in a variety of aerospace applications embracing both airframe and engine components. Aluminum, titanium and nickel based alloys could all be used for the matrix material in appropriate applications where some improvement is sought over the properties offered by unreinforced monolithic material. Typical material targets might be high temperature titanium alloy turbine blades, and high stiffness lightweight aluminum alloy airframe components or aircraft skins.
Conventionally, long fibre reinforced metal matrix composites for structural applications are produced either by consolidation of a fibre/foil or fibre/powder preproduct or else by liquid metal infiltration of a fibre preproduct. These production routes do not lend themselves to the production of metal matrix composites with complex geometry. It is known to use a barrier layer or layers upon ceramic fibres to limit reaction with a matrix metal such as titanium. The prior art also includes proposals to place a single layer of dissimilar material between reinforcement and matrix to achieve a better sharing of load between the two. Neither reaction barrier layers or load transfer layers are intended to impart any structural attribute to the composite other than the effect which they exert on the reinforcement/matrix interface. They do not alter in any way the damage tolerance properties of the matrix as the present invention seeks to do.
In some prior art materials, sheets of monolithic-matrix composites are laminated on a macroscopic scale with sheets of unreinforced materials or dissimilar sheets of metal composites so that the completed laminate has attributes traceable to both constituent sheets. Such laminates have significantly greater interlayer distances than might be desirable for structural considerations alone and they are significantly anisotropic, being the product of laminated sheets.